The Southern Oscillation Index (SOI) is an index that is derived from sea level pressure differences between Tahiti (eastern Pacific) and Darwin, Australia (western Pacific). The SOI corresponds well with changes in ocean temperatures across the tropical eastern Pacific. The negative phase of the SOI represents below-normal air pressure at Tahiti and above-normal air pressure at Darwin. The positive phase of the SOI represents above-normal air pressure at Tahiti and below-normal air pressure at Darwin . Prolonged periods of negative (positive) SOI values coincide with abnormally warm (cold) ocean waters across the eastern tropical Pacific which is indicative of El Nino (La Nina).
Source: www.ncei.noaa.gov/access/monitoring/enso/soi
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